A coaxial cable in a community antenna television (“CATV”) system may be used to provide telephony services in addition to traditional cable television programming. The power supply signal used to power telephony components in such a system is typically generated as a DC signal or an AC sine wave signal at a central office or head end facility. The voltage of the power supply signal is typically about 90 volts. However, as the power supply signal propagates through a CATV system, the waveform may become altered, as the impedance of the various loads that are supplied by this signal can affect the shape of the waveform. Thus, although the power supply signal may be generated as a AC signal having a sinusoidal shape, by the time the signal reaches a particular piece of equipment, such as a VOICE PORT™ device provided by ARRIS™ International, Inc., the sine wave may have mutated into a signal having a shape other than a sinusoid, a triangle wave, for example. In addition, the resulting triangle wave may not be symmetrical, either with respect to the waveform on either side of a vertical axis through the a peak, or with respect to the 0 V horizontal axis (i.e., a DC offset). Thus, the root-mean-square (“RMS”) value cannot be predicted by merely measuring the peak-to-peak voltage of the supply signal.
It is desirable to determine the RMS value of the supply voltage at a VOICE PORT™ or other cable telephony device, as these devices often comprise circuitry that is of the constant-power type. In other words, the telephony circuitry constantly attempts to draw a fixed amount of power from a power supply. If the voltage goes up, the current drawn will typically go down. Conversely, if the supply voltage drops, the current drawn by the constant-power circuitry will rise according to P=V×I. If the supply voltage is monitored, protection circuitry can take sensitive, constant-power circuitry out of service while dangerous voltage levels exist. Dangerous voltage can occur as higher than normal voltage levels, but low voltage levels are typically more dangerous because of the high current that is drawn to maintain a constant power level.
Since heating, and damage-causing overheating, is directly related to the total amount of current that flows through a circuit, monitoring the RMS voltage of a power supply signal more accurately predicts the total power used by a constant-power device, and its internal circuitry, than does the monitoring of peak-to-peak voltage. To monitor the current draw in cable telephony devices, peak detection circuitry has been used. This provides a measure of the peak voltage of a supply power signal. Peak detection technology is adequate for use as long as the power-supply-signal waveform is known, because circuits can be designed to remove a cable telephony circuit from service when the peak supply voltage drops below a predetermined level. For example, if the maximum RMS current that can be safely drawn by a circuit occurs when the RMS voltage is 56.6 V, then protection circuitry could be set to operate when the peak voltage of a power supply signal falls below 80 V (80 V×0.707=56.56 V) for a supply having a sinusoidal waveform. However, for a triangle shaped waveform having a duty cycle of D=1, the RMS voltage would only be 46.2 V for a peak voltage of 80 V. Therefore, although the supply voltage would be acceptable based on the 80 V measured peak voltage, damage could occur because the constant-power telephony circuit would draw higher current to compensate for the lower RMS voltage.
Thus, there is a need for a device that can accurately measure the RMS voltage of a signal regardless of the waveform shape.